Prior to the lime mud reburning kiln, the lime mud having a dry solids content of approx. 25% is thickened normally to a dry solids content of 70-85% in a drum vacuum filter. At the beginning of the filtering process a so-called precoat layer is formed of the lime mud on the surface of the filtering drum; the thickness of the precoat is normally 10-20 mm. A scraper is used to remove the dried lime mud thickened onto the surface of the precoat layer. The dried lime mud falls onto a conveyor and further to a feed screw of the lime mud reburning kiln supplying the dried mud to the kiln.
The surface layer of the precoat becomes clogged while the filtering proceeds and from time to time it must be removed. The technique normally used is to move the automatic scraper gradually inwards following a predetermined program, thus removing the clogged surface layer. After the scraper has moved in several steps closer to the drum the rest of the precoat is removed by-means of air or water, and a new precoat is formed.
All the movements of the scraper described above and the removal of the whole precoat from the surface of the drum cause disturbances in the operation of the lime mud reburning kiln. Every time the clogged surface layer of the precoat has been removed by means of the scraper the filtering is more efficient and the lime mud volume is larger for a moment. Further, also the dry solids content of the lime mud is higher after the precoat has been scraped which results in and calls for changes the running of the lime mud reburning kiln. When the whole precoat is renewed at the same time the lime mud is damp and the supply of the mud into the kiln is stopped altogether for a moment which naturally causes a break in the operation of the kiln. Typically, the precoat is renewed once a shift, or correspondingly three times in 24 hours. In the long run, also the wire is so badly clogged that it must be washed either with acid or with a high-pressure washer. This operation also causes a long disturbance or interrupts the production of the lime mud reburning kiln.
The arrangement disclosed in U.S. Pat. No. 5,149,448 reduces the disadvantages and problems of the known techniques described above by removing a strip at a time from the filtering layer by means of a liquid jet reciprocating continuously in the longitudinal direction of the drum. This has been effected by installing under the scraper one nozzle or several nozzles continuously reciprocating in the longitudinal direction of the drum. The high-pressure nozzles, for example two in number, have been provided under the scraper so that the purpose of one of the nozzles is to break the precoat layer and to drop it to the filter vat after which the second nozzle is used to wash the filter cloth clean. The pressure of the water sprayed from the washing nozzles is approx. 5-100 bar which gives a very good cleaning result. The nozzle disperses the jets so that the precoat is removed and the wire washed from a strip of approx. 10-50 in width at a time.
According to the arrangement of the above US patent, the washing nozzles have been installed in a rack designed for this purpose for moving the nozzles sideways. The nozzles are moved continuously to and fro in the longitudinal direction of the drum thus continuously renewing the precoat and cleaning the filter cloth. The advantages provided by this kind of continuous high-pressure water scraping are:
the dry-solids content of the lime mud from the lime mud filter to the lime mud reburning kiln is continuously uniform which allows smooth running of the kiln; PA1 change of the precoat is not needed as the precoat is renewed continuously a strip at a time; thus disturbances in the operation of the kiln due to a change of the precoat are avoided; PA1 it is not necessary to interrupt the filtering operation for a wash of the wire. PA1 partial removal of the precoat with a gentle liquid jet; PA1 easier removal of the whole precoat layer because the precoat is not hard; PA1 intermittent use of the high-pressure jets PA1 the location of the washing apparatus is optimal in view of installation and maintanence; PA1 the direction of the washing nozzles is optimal in view of their operation and staying clean.
In the arrangement of said US patent the nozzle/nozzles are located under the scraper removing dried lime mud from the drum; thus the location is as inconvenent as possible in view of the installation and maintenance of the nozzles. Also, lime mud splashes from nozzles located under the scraper may impede the operation of the scraper. Further, because of the operation way of the nozzles and their direction determined by the operation way, the splashes caused by the removal of the precoat impinge on the nozzles and dry as deposits on the structures preventing the movement of the nozzle rack. Further, the precoat is hardest under the scraper because it has been longest under the influence of the pressing partial vacuum.